A fuel cell is an energy conversion device that consists essentially of two opposing electrodes, an anode and a cathode, ionically connected together via an interposing electrolyte. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or a hydrocarbon (e.g., methanol), to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell system can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cell systems employing methanol as the fuel supplied to the anode (also commonly referred to as a “Direct Methanol Fuel Cell” (DMFC) system), the electrochemical reactions are essentially as follows: first, a methanol molecule's carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule's oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air (supplied to the cathode) is likewise simultaneously reduced at the cathode. The ions (protons) formed at the anode migrate through the interposing electrolyte and combine with the oxygen at the cathode to form water. From a molecular perspective, the electrochemical reactions occurring within a direct methanol fuel cell (DMFC) system are as follows:
Anode:CH3OH + H2O → 6H+ + 6e− + CO2E0 = 0.04 Vvs. NHE(1)Cathode:                    3        2            ⁢              O        2              +          6      ⁢              H        +              +          6      ⁢              e        -              →      3    ⁢          H      2        ⁢    O  E0 = 1.23 Vvs. NHE(2)Net:                    CH        3            ⁢      OH        +                  3        2            ⁢              O        2              →            2      ⁢              H        2            ⁢      O        +          CO      2      E0 = 1.24 Vvs. NHE(3)The various electrochemical reactions associated with other state-of-the-art fuel cell systems (e.g., hydrogen or carbonaceous fuel) are likewise well known to those of ordinary skill in the art of fuel cell technologies.
With respect to state-of-the-art fuel cell systems generally, several different configurations and structures have been contemplated—most of which are still undergoing further research and development. In this regard, existing fuel cell systems are typically classified based on one or more criteria, such as, for example, (1) the type of fuel and/or oxidant used by the system, (2) the type of electrolyte used in the electrode stack assembly, (3) the steady-state operating temperature of the electrode stack assembly, (4) whether the fuel is processed outside (external reforming) or inside (internal reforming) the electrode stack assembly, and (5) whether the reactants are fed to the cells by internal manifolds (direct feed) or external manifolds (indirect feed). In general, however, it is perhaps most customary to classify existing fuel cell systems by the type of electrolyte (i.e., ion conducting media) employed within the electrode stack assembly. Accordingly, most state-of-the-art fuel cell systems have been classified into one of the following known groups:                1. Alkaline fuel cells (e.g., electrolyte is KOH);        2. Acid fuel cells (e.g., electrolyte is phosphoric acid);        3. Molten carbonate fuel cells (e.g., electrolyte is 63% Li2CO3/37% K2CO3);        4. Solid oxide fuel cells (e.g., electrolyte is yttria-stabilized zirconia);        5. Proton or ion exchange membrane fuel cells (e.g., electrolyte is NAFION).        
Although these state-of-the-art fuel cell systems are known to have many diverse structural and operational characteristics, such conventional systems nevertheless share common characteristics with respect to their reactant delivery and reaction product removal systems. More specifically, conventional state-of-the art fuel cell systems have fluid transport and channel structures designed to predominantly transport reactants (i.e., a fuel and/or oxidant) via “passive diffusion” to the diffusion boundary layer associated with a catalytic film/layer that has been deposited on an electrode (positive or negative) structure. As is appreciated by those skilled in the art, the catalytic film/layer associated with conventional fuel cell electrodes is a substantially planar region where the electrochemical oxidation-reduction reactions between chemical species occur, and the diffusion boundary layer is the region very close to the electrode surface where, due to side-wall frictional forces, flow velocity tends to zero and mass transport by convection is necessarily negligible. In general, conventional fluid transport and channel structures that rely predominantly on a passive diffusion transport mechanism provide less than optimal mass transfer kinetics, especially in the context of direct liquid feed fuel cells.
As is appreciated by those skilled in the art, mass transfer in a electrochemical fuel cell (ie., the movement of material from one location to another) arises from differences in electrical and/or chemical potentials at separate locations, and/or from movement of a volume element of a fluid (gas or liquid). Accordingly, the modes of mass transfer are generally understood to be as follows: (1) migration, or the movement of a charged particle under the influence of an electric field such as an electrical potential gradient; (2) diffusion, or the movement of a species under the influence of a gradient of chemical potential such as a concentration gradient, and (3) convection, or fluid flow that occurs because of natural convection (ie., convection caused by density gradients) and/or because of forced convention (i.e., convection caused by hydrodynamic transport). All three modes of mass transfer may be present, to varying degrees, within existing electrochemical fuel cell systems.
As is further appreciated by those skilled in the art, most conventional DMFC systems generally consist of, among other things, a series of membrane electrodes assemblies (MEAs) configured into a stack, wherein each individual MEA further consists of a two opposing electrodes each having catalytic “active” regions. During operation, the catalytic active regions of such conventional MBAs are exposed, either directly or indirectly, to an adjacently flowing fluid stream (contained within an adjacently positioned fluid removal or delivery flow channel). Importantly, because the catalytic active regions of conventional fuel cell systems are substantially planar in character (e.g., a carbon-fiber sheet or layer having affixed or embedded catalyst particles), and because such active regions are positioned adjacent to a flowing fluid during operation (e.g., a fluid flowing in one or more parallel grooves machined into a graphite or aluminum block), the mass transport of reactants (especially gaseous reactants) to the outer boundary of the diffusion layer of the catalytically active electrode surface is predominantly “diffusion” mass transfer. Stated somewhat differently, incoming reactant fuel must laterally diffuse from diverse locations within a non-catalytically active porous region in order to reach a thin catalytically active proton exchange layer; byproducts of reaction must then laterally diffuse away from the proton exchange layer as they are simultaneously axially convected toward the outlet channel.
For purposes of example, at higher current densities, the poor performance characteristics of conventional DMFC systems have been attributed to diffusion mass transfer limitations associated with fuel and oxidant delivery.
In addition to the foregoing, several different configurations and structures have been contemplated for direct liquid feed fuel cell systems such as, for example, a direct methanol electrode structure having a solid polymer electrolyte (SPE). Because such polymer electrolytes are typically cast as solid membranes, this type of electrode assembly is commonly referred to as a “membrane electrode assembly” (MEA). A typical MEA consists essentially of a proton conducting membrane (i.e., the solid polymer electrolyte) sandwiched between two platinum coated electrode structures. A significant problem, however, with DMFC systems having MEAs is a phenomenon known as “methanol cross-over.” methanol in conventional DMFC systems has a tendency to cross-over from the anode to the cathode via diffusion (i.e., it migrates through the electrolyte), where it adsorbs onto the cathode catalyst and reacts with oxygen from air resulting in a parasitic loss of methanol fuel and concomitant reduction in fuel cell voltage. Indeed, performance losses of 40-100 mV at a given current density have been observed at the cathode of DMFC systems utilizing a direct methanol feed (Potje-Kamloth et al., Abstract No. 105, Extended Abstracts 92-2, “Fall Meeting of the Electrochemical Society” (1992), Kuver et al., J Power Sources 52:77 (1994)).
Conventional attempts for reducing methanol cross-over in DMFC systems having MEAs include structural modifications of the central solid polymer membrane. Exemplary in this regard are the MEAs disclosed in (1) U.S. Pat. No. 4,664,761 to Zupancic et al. (discloses proton-conducting membrane made of an interpenetrating polymer network); (2) U.S. Pat. No. 5,672,438 to Banarjee et al. (discloses proton-conducting laminated membrane); and (3) U.S. Pat. No. 5,919,583 to Grot et al (discloses proton-conducting membrane that includes an inorganic filler). Although the various MEA designs disclosed in these patents are able to reduce methanol cross-over to some degree, they nevertheless still have relatively high methanol permeabilities.
Other attempts for reducing methanol cross-over include the incorporation of a metallic barrier layer into the electrode assembly. Exemplary in this regard are the metal hydride barrier layers disclosed in (1) Pu et al., “A Methanol Impermeable Proton Conducting Composite Electrolyte System,” J Electrochem. Soc., 142(7):119-120 (July 1995) (discloses a three-layered laminar electrolyte consisting of a palladium foil layer sandwiched between two polymeric electrolytes); (2) U.S. Pat. No. 5,759,712 to Hockaday (discloses a semi-permeable plastic electrode structure having a top palladium membrane that contains numerous swellable voids); and (3) U.S. Pat. No. 5,846,669 to Smotkin et al. (discloses a hybrid electrolyte system consisting of an acid electrolyte, a base electrolyte, and an interposing palladium foil layer). Although the various MEA designs disclosed in these patents are better able to reduce methanol cross-over than other conventional designs, they too are also less than optimal because of problems caused by the poor ability of such foils to tolerate mechanical stresses (from hydration cycling effects). In short, these state-of-the-art hydrogen permeable metallic blocking layers are known to experience significant problems with cracking and/or delamination.
Although significant progress has been made with respect to these and other fuel cell system problems, there is still a need in the art for improved fluid transport and flow channel passageways associated with fuel cell electrode structures and fuel cell electrode stack assemblies, as well as improved direct liquid feed fuel cell systems. The present invention fulfills these needs and provides for further related advantages.